Feature request: use caplog with custom formatter

My application uses a quite heavily customized logging formatter.
Right now, the logging plugin only allows to specify the formats for the formatter, not completely replacing it.
It would be nice if there was a clean way to provide a custom formatter.

This is the workaround I use for now, patching the plugin class on the module:

from _pytest import logging as pytest_logging

def pytest_configure(config):
    config.pluginmanager.get_plugin('logging').LoggingPlugin = MyLoggingPlugin

class MyLoggingPlugin(pytest_logging.LoggingPlugin):
    def __init__(self, config):
        super().__init__(config)
        self.formatter = MyCustomFormatter()

Start at _pytest.logging.LoggingPlugin and the pytest_configure workaround shown in the issue. Trace how the current formatter configuration is exposed, then define a supported way for applications to provide a custom formatter without replacing the plugin class. Done means the custom formatter can be supplied through the supported interface.